What I understand, its not posseble to use that feuture without reinstalling and change docroot to /var/www ? Or recompile the suexec?

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It depends on your distribution. On Fedora, Mandriva, Debian and Ubuntu the suExec docroot is /var/www, on SuSE it's /srv/www.
If your ISPConfig web root is different from suExec's docroot, and you want to use suExec, then you must either uninstall and then reinstall ISPConfig (an update doesn't help) or recompile suExec...